Frequency modulated carrier detector



Feb. 17, 1942. s, -r

FREQUENCY MODULATED CARRIER DETECTOR Filed April 10, 1941 2 Sheets-Sheetl INVENTOR .9

BY %i"2" ATTORNEY Feb. 17, 1942.

S. HUNT FREQUENCY MODULATED CARRIER DETECTOR Filed April 10, 1941 2Sheets-Sheet 2 elgl VENTOR gymaar BY flaunt ATTbRNEY circuit of FigPatented F b. 17, 1942 UNITED STATES PATENT OFFICE;

FREQUENCY MODULATED CARRIER DETECTOR H Seymour Hunt, Flushing, N. Y.,assignor to Radio Corporation of Amcrica, a corporation of DclawareApplication April- 10, 1941, Serial No. 387,829

9 Claims.

My present invention relates to detectors for frequency modulatedcarrier waves, and more especially to detectors of the multi-grid' typecapable of deriving modulation components from angularvelocity-modulated carrier waves.

' It has been shown. in the past that if angular velocity-modulatedcarrier waves. are applied in phase quadrature relation to.spacedelectrodes in a. common electron stream to an output electrode,there will be produced. in the output ch- 3 cuit modulation voltagecorresponding to the modulation-representative deviations of thecenvoltage at one grid-in phase quadrature with the [carrier-voltage ata second grid.

{Still another ob ect of my invention is .to improve frequency modulatedcarrier wave dejtectors of the wherein wave voltages are appliedin phasequadrature to spaced grids of a multi-grid tube, andtheimprovementresiding ina tuned device employed in association with oneofthe grids for producing the phase quadrature relation.

connection with the drawings in which I have indicated diagrammaticallyseveral circuitor- 'ganizations whereby my invention may be carried intoeife'ct.

Inthe drawings: Y, Fig. lshows a circuit embodying the invention, Fig. 2illustrates a modification of the cathode Figs. 3 and 4 show'respectively ditional modifications,

Figs. 5 and 6 show two different embodiments,

Fig. '7 shows still another form of the invention.

Referringnow, to Fig. 1 of the accompanying I drawings, wherein likereference characters in "the difierentfig'ures designate similar circuitelements,the tube 1 may be a multi-grid tube ofthe6A8 type. The grid 2is the signal voltage input grid, and'it'is connected to the cathode,

different ad .30 The novel features which I believe to be char-.acteristi'c of my invention are set forth in particularity in theappended claims; the invention itself,- however, as to both itsorganization and method of operation will best beunderstood by{reference to the following description taken in of tube I througha-path comprising, coil 3 and resistor 5. The coil 3. is shunted bycondenser 4,, and the two are tuned to the operating intermediatefrequency (I. F.) of the system, assuming the latter is asuperheterodyne receiver. The I. F. may have a value of 4.2v mc.(megacycles), where the FM? (frequency modulated carrier wave) bandcovers 42 to 50 mo. As is well known, there are impressed upon circuit1' frequency modulated carrier waves whose center frequency is-of the4.2 mc. value. The circuits prior to tuned circuit 1 may include asignal collecting circuit, a converter, one or more I. F. amplifiers anda limiter stage. 7

As is well known to those skilled in the art, the function of thelimiter is to. eliminate amplitude modulation effects'in thecarrier. Thelimiter tube may be a saturated amplifier, and the circuit 1 will belocated in the plate circuit of'the limiter tube. vHence there will. bedelivered to input circuit. 3- 4 frequency variable waves of constantamplitude. It is, of course, well known that in frequency modulationtransmission the modulating signal amplitude ap: pears as a frequencydeviationof the carrier, or center, frequency, while the modulating frefquencies themselves correspond to the rate;o f deviation of thecenter frequency. The function 'of tube and its associated circuits isto derive from the frequency-variable wave the modulation voltagewhichcorresponds to the original modulating signals.

The grid 1 of tube l is located betweena pair oflpositively biasedscreen grids 8 and 9-, while the output electrode, or plate, H) isconnected to the positive terminalof a source of direct current by aload resistor II. The plate end of resistor H is by-passedto'ground bycondenser 12 which has a low impedance to I. F. currents. Modulationvoltage developed across'resistor II is transmitted to a succeedingaudio frequency utilization network by condenser l3' which has'a lowimpedance to modulation frequencyvoltage.

The cathode of tube I is connected to ground throu'gha path includingjcoil M 'andby-passed bias resistor If. ,The dotted rectangle Itenjcloses the leads n and is, the latter bann rminated by-the seriestermination M. The network within rectangle I6 is a one-quarter wavelength transmissionline, and functions, as a phase shifting network. Thegrid 1 is connected by lead ZD'to' the junction of resistor 15 and coili l. The .resistorv 5 develops'thereacrossa unidirectional voltage whichmaybe utilized' ffor automatic volume controliAvC), and the lead 2| isconnected to the grid end of resistor through a filter resistor; thevoltage transmitted over lead 2| is to be understood as being applied tothe grid circuits of amplifier tubes preceding the limiter stage.Condenser 6 acts as an I. F. by-pass condenser across resistor 5.

It has been shown in the past that when frequency modulated carrierwaves of the same center frequency are applied to grids 2 and 'l insubstantially phase quadrature, then there will be developed modulationvoltage across load resistor II. In other words, the circuit functionsas a demodulator. Of course, where the bypassed resistor 5 is insertedin the grid circuit of tube I then there will be provided furtherlimiting action in the grid circuit of the demodulator. In that case aspecial limiter stage prior to the input circuit of detector tube I maybe dispensed with. Those skilled in the art know that the onequarterwave length transmission line in rectangle l6 produces a phase shiftthereacross which is 90 degrees. Thus, the voltages on grid 2 and grid 1differ 90 degrees in phase. The line impedance at the cathode end oflead l'l-l8 is very low and the radio frequency voltage drop at thatpoint is very small. At the terminating endof the line the impedance ishigh andthe radio frequency voltage dropis high. In other words, thenetwork in rectangle l6 may be conconnected to the cathode in Fig. 1 maybe returned to ground. This is shown in Fig. 2. In this case grid 2 hasdegeneratively applied to it radio frequency voltage developed acrossl4. However, there is but small degeneration applied to grid 2 comparedto a large voltage developed for grid 1 by the resonant rise due to thetransmission line in the cathode circuit of the tube.

Another method of producing a 90 degrees phase shift between thefrequency modulated voltages applied to grids 2 and 1 is shown in Fig.3.In this figure the cathode of tube l is connected to ground through acoil 22 which is shunted by condenser 23. Circuit 2223 is tuned tothecenter frequency Fe of the applied FM waves. A small resistor 24 isplaced in series with condenser 23 to provide quadrature voltagethereacross. Grid 1 is connected to the junction of condenser 23andresistor 24 and takes off the quadrature voltage across the latter. Inthis way FM voltage developed across the cathode network of tube l isapplied to grid 1 in phase quadrature with the FM voltage applied togrid 2 by input circuit 3-4.

' In'Fig. 4 there is shown still another modification. This arrangementdiffers from that shown .in Fig. 3 in the arrangement of the cathodephase shifting network. Here coil 22' and condenser 23 are arranged inseries between the cathode and ground. The series path 22-23' is seriestuned to Fe, the junction of the series elements In shunt with condenser23' is connected radio frequency choke coil 25in series with resistor26. The radio frequency current through condenser 23' is 90 degrees outof phase with the voltage across tuned circuit 2223'. Thus, voltageapplied to grid 1 is degrees out of phase with voltage at grid 2.

The circuit modification in Fig. 5 diifers from that shown previously inthe other figures in that the grid 7 is connected to ground through apiezo-electric crystal element which is tuned to F0. Thus, the numeral30 designates the crystal which is tuned or cut to Fe, those skilled inthe art being fully aware of the manner of mounting the crystal betweenappropriate electrodes or crystal holder elements. One of the crystalelectrodes is connected to grid 1, while the opposite one is connectedto ground. The electrical equivalent circuit of the crystal would appearas an inductance, capacity and resistance, all constants of the crystal,arranged in series between grid 1 and ground, whereas the capacity ofthe crystal holder would be arranged in shunt with the aforesaid seriesconstants. These equivalent components may be made to appear as a tunedcircuit.

In Fig. 6 is shown another modification wherein the piezo-electricresonator 30 of Fig. 5 is replaced by a mechanical resonant line. Thelatter comprises an outer metallic tube 40 having one end thereofgrounded, whereas there is disposed concentrically within the tube 40 asecond metallic tube 4| connected to grid 1. Of course, the grid 1 willbe tapped to a proper point on the inner tube 4|. The mechanicalresonator 40-4I is adjusted to tune to Fe.

It often happens that it is desirable to arrange an FM detector so thatit will deliver a pair of audio voltages degrees out of phase whereby apush-pull audio amplifier may be driven. In Fig. 7 there is shown suchan arrangement. Here the tube I has its grid I connected to groundthrough the coil 50 of resonant circuit JO-5|. The latter circuit istuned to Fe, and develops across it FM voltage which is in phasequadrature with. the voltage applied to grid 2. Screen grids 8 and 9 areconnected to the positive terminal of a direct current source through aload resistor H, the latter being shunted by an I. F. by-pass condenser12'. A modulation voltage developed across plate load resistor II istransmitted through condenser [3 to the grid of one of a pair ofpush-pull connected'amplifier tubes, while the voltage developed acrossresistor H is transmitted through condenser l3 to the opposite push-pullamplifier. As the FM voltage applied to grid 2 has its instantaneousfrequency shifted above and below Fc more or less voltage is built upacross circuit 5lI-5l by virtue of space charge coupling between grids 2and I. The mutual conductance of tube I changes as the voltage of grid 1changes, When more plate current flows, less screen grid current flowsand vice versa. This is, of course, the proper polarity sense for theaudio voltages developed across resistors H and ll. of course, the phasequadrature voltage for grid 1 may be developed by using any of thecathode load networks shown in Figs. 1, 2 and 3 and 4 instead of relyingupon space charge coupling of the detector tube.

The following explanation of the operation of Fig. 7 is equallyapplicable to Figs. 5 and 6. Electrons are first attracted by thepositive screen 8. Some go through the open mesh of the screen and areattracted by the positive plate. Due to grid 1 being less positive thanthe screen 8 many electrons are slowed down in speed. A cloud, or pile,ofelectrons gather in front of grid 1. This is the virtual cathode. Theplate draws electrons from the virtual cathode. The

virtual cathode waxes and wanes in relation to the voltage applied togrid 2. This motion of electrons causes a current to flow in grid 1 andtuned circuit LDC5l. At resonance L5'o"-C'51 looks like a resistor, andit has been found that the voltage across Lao-C51 at resonance is 90degrees out of phase with the voltage applied to grid 2. The actualplate current flowingis proportional to the voltage on grid 2 and thevoltage on grid 1. As the carrier shifts L5o-C5i is not a resistor anylonger, but either a capacity or inductance depending upon whether theapplied frequency is above or below the tuned frequency of Law-C51 Thus,the voltage across L5o--C51 is either more or less than 90 degrees outfrom the applied grid 2 voltage. Thus, as the modula- I tioncauses thevoltage of grid 2 to vary in frequency the plate current varies in stepwith the modulation, and provides the audio output.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular or anizationssnown and described, but that many mocuncations may be made withoutdeparting from the scope of my invention, as setforth in the appendedclaims.

What I claim isi 1. In an angular velocity modulated carrier wavedetection network, a tube provided with at least a cathode, an outputelectrode and a pair of spaced control grids in the electron streambetween the cathode and output electrode, a modu lated carrier inputcircuit connected between one control grid and the cathode, an outputload element across whichis developed modulation voltage connected tothe output electrode, a phase shifting network connected in the spacecurrent path of said tube between said cathode and a point of relativelyfixed alternating potential, said shifting network developing modulatedcarrier voltage which is in phase quadrature with the voltage at saidinput circuit, and means for applying said quadrature voltage to thesecond control grid of said tube.

2. In an angular velocity modulated carrier wave detection network, atube provided with at least a cathode, an output electrode and a pair ofspaced control grids in the electron stream .between the cathode andoutput electrode, a

modulated carrier input circuit connected between one control grid andthe cathode, an output load element'across which is developed modulationvoltage connected to the output electrode,

a phase shifting network connected in the space current path of saidtube between said cathode and a point of relatively fixed alternatingpotential, said shifting network developing modulated carrier voltagewhich is in phase quadrature with the voltage at said input circuit, andmeans for applying said quadrature voltage to the second control grid ofsaid tube, said phase shifting network consisting of a quarter wavelength transmission line.

3. In an angular velocity modulated carrier wave detection network, atube provided with at least a cathode, an output electrode and a pair ofspaced control grids in the electron stream between the cathode andoutputelectrode, a modulated carrier input circuit connected between onecontrol grid and the cathode, an output load element across which isdeveloped modulation voltage connected to the output electrode, a phaseshifting network connected in the space current wave detection network,a tubeprovided with at a least a cathode, anoutput electrode and a pairof spaced control grids in the electron stream between the cathode andoutput electrode, a modulated carrier input circuit connected betweenone control grid and the cathode, an output load element across which isdeveloped modulation voltage connected to the output electrode, a phaseshifting network connected in the space current path of said tubebetween said cathode and a point of relatively fixed alternatingpotential, said shifting network developing modulated carrier voltagewhich is in phase quadrature with the voltage at said input circuit, andmeans for applying said quadrature voltage to the second control grid ofsaid tube, said phase shifting network consisting of a coil andcondenser series resonant to the center frequency of the waves of saidinput circuit, and said second control grid being connected to thejunction of said coil and condenser.

5. In an angular velocity modulated carrier wave detection network, atube provided with at least a cathode, an output electrode and a pair ofelectrodes arranged to control the electron stream between the cathodeand output electrode, a modulated carrier input circuit connectedbetween one control electrode and the cathode, an output load elementacross which is developed modulation voltage connected to the outputelectrode, a reactive phaseshifting network connected between thecathode and a point of relatively fixed alternating potential anddeveloping modulated carrier voltage which is in phase quadrature withthe voltage at said input circuit, and means for connecting a point ofsaid shifting network to the second control electrode of said tube.

6. In an angular velocity modulated carrier wave detection network, atube provided with at least a cathode, a plate and at least two controlgrids in the electron stream between the cathode and plate, a modulatedcarrier input circuit connected between one control grid and thecathode, an output resistor element across which is developed modulationvoltage connected to the plate, a phase shifting network connected inthe space current path of said tube between said cathode and ground,said shifting network consisting of a transmission line of a lengthsuch.- as to develop modulated carrier voltage which is in substantialphase quadrature with the voltage at said input circuit, and means forapplying saidquadrature voltage to the second controlgrid of saidtube-..

'7. In an angular velocity modulated carrier Wave detection network, atube provided with at least a cathode, an output electrode and at leasta pair of spaced control grids in the electron stream between thecathode and output electrode, a modulated carrier input circuitconnected betweenone control grid and the cathode, an output loadelement across which is developed modulation voltage connected to theoutput electrode, a phase shifting network connected in the spacecurrent path of said tube between said cathode and a point of relativelyfixed alternating potential, said shifting network including an openended transmission line of a wave length chosen to develop modulatedcarrier voltage which is in substantial phase quadrature with thevoltage at said input circuit, and means for applying said quadraturevoltage to the second control grid of said tube.

8. In an angular velocity modulated carrie wave detection network, atube provided with at least a cathode, an output electrode and a pair ofspaced control grids in the electron stream between the cathode andoutput electrode, a modulated carrier input circuit connected betweenone control grid and the cathode,

an output load element across which is developed modulation voltageconnected to the output electrode, a phase shifting network connected inthe space current path of said tube between said cathode and a point ofrelatively fixed alternating potential, said shifting network developingmodulated carrier voltage which is in substantial phase quadrature withthe voltage at said input circuit, and means for applying saidquadrature voltage to the second control grid of said tube, said phaseshifting network comprising a coil and condenser arranged as a parallelresonant circuit tuned to the center frequency of the carrier wave ofsaid input circuit, a resistor in series with the condenser, saidapplying means including a connection between the second grid and thejunction of the resistor and condenser.

9. In an angular velocity modulated carrier wave detection network, atube provided with at least a cathode, an output electrode and a pair ofspaced control grids in the electron stream between the cathode andoutput electrode, a modulated carrier input circuit connected betweenone control grid and the cathode, an output load element across which isdeveloped modulation voltage connected to the output electrode, a phaseshifting network connected in the space current path of said tubebetween said cathode and a point of relatively fixed alternatingpotential, said shifting network developing modulated carrier voltagewhich is in phase quadrature with the voltage at said input circuit,means in circuit with said one grid for limiting the response of thenetwork to carrier amplitude variation, and means for applying saidquadrature voltage to the second control grid of said tube.

SEYMOUR HUNT.

